Methods of forming assemblies displaying differential negative resistance

ABSTRACT

The invention includes a device displaying differential negative resistance characterized by a current-versus-voltage profile having a peak-to-valley ratio of at least about 9. The invention also includes a semiconductor construction comprising a substrate, and a first layer over the substrate. The first layer comprises Ge and one or more of S, Te and Se. A second layer is over the first layer. The second layer comprises M and A, where M is a transition metal and A is one or more of O, S, Te and Se. A third layer is over the second layer, and comprises Ge and one or more of S, Te and Se. The first, second and third layers are together incorporated into an assembly displaying differential negative resistance. Additionally, the invention includes methodology for forming assemblies displaying differential negative resistance, such as tunnel diode assemblies.

TECHNICAL FIELD

The invention pertains to semiconductor constructions, such as, for example, assemblies displaying differential negative resistance. The invention also pertains to methods of forming assemblies displaying differential negative resistance. In particular aspects, the assemblies displaying differential negative resistance are tunnel diode assemblies.

BACKGROUND OF THE INVENTION

Assemblies displaying differential negative resistance, such as tunnel diode assemblies, can be used as components in various types of circuits, including, for example, logic circuits, and microwave oscillators. It can therefore be desired to incorporate assemblies displaying differential negative resistance into integrated circuit structures.

Various devices can be characterized in terms of the current passing through the devices relative to the amount of voltage applied to the devices. Some devices, such as, for example, tunnel diodes, will have an operating region in which an ohmic relationship of current as a function of voltage breaks down, and where a tunneling effect begins. A plot of current-versus-voltage for the devices will have a region referred to as the differential negative resistance (DNR) region, characterized by a non-ohmic relationship of current to voltage, and comprising a maximum in current value (typically referred to as a peak current value), and a minimum in current value (typically referred to as a valley current value).

The ratio of the current at the peak to the current at the valley can be utilized as a figure-of-merit for a device displaying differential negative resistance. Higher peak-to-valley ratios are considered to indicate better-quality devices. Presently, the highest peak-to-valley ratios obtained for integrated circuit tunnel diode devices are less than 8. It would be desirable to develop better devices displaying differential negative resistance which can be incorporated into integrated circuits.

It would also be desirable to develop methodology for incorporating such devices into integrated circuit constructions.

SUMMARY OF THE INVENTION

In one aspect, the invention pertains to a device displaying differential negative resistance characterized by a current-versus-voltage profile having a peak-to-valley ratio of at least about 9.

In one aspect, the invention pertains to a semiconductor construction comprising a substrate, and a first layer over the substrate. The first layer comprises Ge and one or more of S, Te and Se. A second layer is over the first layer. The second layer comprises M and A, where M is a transition metal and A is one or more of O, S, Te and Se. A third layer is over the second layer, and comprises Ge and one or more of S, Te and Se. The first, second and third layers are together incorporated into an assembly displaying differential negative resistance.

In one aspect, the invention encompasses a semiconductor construction which includes a substrate and an assembly displaying differential negative resistance supported by the substrate. The assembly comprises a material which includes Ge, a transition metal, and one or more of S, Te and Se.

The invention also encompasses methodology for forming the constructions and structures described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below with reference to the following accompanying drawings.

FIG. 1 is a diagrammatic, cross-sectional view of a semiconductor wafer fragment at a preliminary processing stage in accordance with an aspect of the present invention.

FIG. 2 is a view of the FIG. 1 fragment shown at a processing stage subsequent to that of FIG. 1.

FIG. 3 is a view of the FIG. 1 fragment shown at a processing stage subsequent to that of FIG. 2.

FIG. 4 is a view of a semiconductor wafer fragment shown at a processing stage subsequent to that of FIG. 1, and in accordance with a second aspect of the invention.

FIG. 5 is a view of the FIG. 4 fragment shown at a processing stage subsequent to that of FIG. 4.

FIG. 6 is a view of the FIG. 4 fragment shown at a processing stage subsequent to that of FIG. 5.

FIG. 7 is a view of the FIG. 4 fragment shown at a processing stage subsequent to that of FIG. 6.

FIG. 8 is a graph of current-versus-voltage for an exemplary device displaying differential negative resistance formed in accordance with an aspect of the present invention.

FIG. 9 is a graph of current-versus-voltage for the so-called first tunnel region of an exemplary device displaying differential negative resistance formed in accordance with an aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first aspect of the invention is described with reference to FIGS. 1-3. Referring initially to FIG. 1, a semiconductor fragment 10 is illustrated at a preliminary processing stage. Fragment 10 comprises a substrate 12. Substrate 12 can comprise, for example, monocrystalline silicon. To aid in interpretation of the claims that follow, the terms “semiconductive substrate” and “semiconductor substrate” are defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.

A layer 14 is formed over substrate 12. Layer 14 is an optional insulative material, and can comprise, for example, silicon dioxide, silicon nitride, borophosphosilicate glass (BPSG), and/or other insulative materials. Additionally, various conductive structures (not shown) can extend into or through insulative material 14. Such conductive structures can comprise part of an integrated circuit supported by semiconductor substrate 12.

A conductive material 16 is formed over insulative material 14. Conductive material 16 can comprise, for example, various metals, such as, for example, one or more of silver, tungsten, titanium and platinum. In particular aspects, conductive material 16 can comprise titanium nitride, either alone, or in conjunction with one or more metals and/or metal alloys. Accordingly, conductive 16 can comprise, consist essentially of, or consist of one or more of Ag, W, Ti, TiN and Pt. Additionally, layer 16 can comprise a conductively-doped semiconductive material. Layer 16 can be referred to as a bottom electrode in the discussion that follows.

Referring to FIG. 2, a layer 18 is formed over bottom electrode 16. Layer 18 can be referred to as a first layer of an assembly displaying differential negative resistance (such as a tunnel diode assembly), and can comprise Ge and one or more of S, Te and Se.

In particular applications, layer 18 will comprise, consist essentially of, or consist of Ge and Se. For instance, layer 18 can comprise, consist essentially of, or consist of Ge_(x)Se_((1-x)), such as, for example, Ge_(0.4)Se_(0.6) (also referred to as Ge₄₀Se₆₀).

Layer 18 can be formed by, for example, either sputter deposition or evaporative deposition. If layer 18 is formed by sputter deposition, it can be formed from a target comprising both Ge and Se, and comprising the Ge and Se in a ratio which is identical to, or approximately, the desired ratio of the elements in layer 18. Accordingly, if the desired ratio of Ge₄₀Se₆₀ is to be formed in layer 18, the layer can be sputter-deposited from a target comprising approximately 60 parts Se for every 40 parts Ge. The target can be a mosaic target.

If layer 18 is formed by evaporative deposition, the layer can be formed by providing a source of Ge and a source of Se within a reaction chamber, and subsequently evaporating Ge and Se from the sources at appropriate rates to form the layer 18 having the desired stoichiometry of Ge and Se.

In the shown embodiment, layer 18 is physically against bottom electrode 16. It is to be understood, however, that optional intervening materials (not shown) can be provided between layers 16 and 18.

A layer 20 is formed over layer 18. Layer 20 can be referred to as a second layer of a construction displaying differential negative resistance. Layer 20 comprises M and A, where M is a transition metal and A is one or more of O, S, Te and Se. The M and A are typically present in a stoichiometry of M_((2+Δ))A, where Δ is greater than 0. The transition metal (M) can be, for example, a metal selected from Group IB of the periodic table (also referred to as Group 11 under the so-called new notation), and accordingly can be copper or silver, for example. In particular embodiments, second layer 20 will comprise, consist essentially of, or consist of Ag_((2+Δ))A, or Cu_((2+Δ))A, where A is O, S, Te, and/or Se.

Layer 20 can be formed by, for example, either sputter deposition or evaporative deposition. In the shown aspect of the invention, layer 20 is physically against an upper surface of layer 18. Layer 20 can be formed by directly depositing a layer comprising M_((2+Δ))A, or by depositing a layer which is subsequently converted to M_((2+Δ))A. For instance, layer 20 can be formed by initially depositing a layer of Ag₂Se, and subsequently diffusing additional Ag into the layer from regions proximate the layer to convert the layer to Ag_((2+Δ))Se.

A layer 22 is formed over layer 20. Layer 22 can be referred to as a third layer of an assembly displaying differential negative resistance, and can comprise the same materials described previously for first layer 18. Accordingly, layer 22 can comprise, consist essentially of, or consist of Ge and one or more of S, Te and Se. Layer 22 can be formed by, for example, either sputter deposition or evaporative deposition. In the shown aspect of the invention, layer 22 is physically against an upper surface of second layer 20.

An electrically conductive layer 24 is formed over layer 22. Layer 24 can comprise the same metal (M) as is utilized in layer 20. Alternatively, layer 24 can comprise a different metal than is utilized in layer 20. In particular applications, layer 24 will comprise, consist essentially of, or consist of one or more transition elements. In an exemplary application, layer 24 will comprise, consist essentially of, or consist of silver. Layer 24 is typically very thin, and can comprise, for example, a thickness less than 300 angstroms, and in particular applications a thickness less than 50 angstroms. Conductive material 24 is physically against an upper surface of material 22 in the shown aspect of the invention.

A conductive material 26 is formed over layer 24. Conductive material 26 can be referred to as a top electrode of an assembly displaying differential negative resistance. Conductive material 26 can comprise identical materials as described above for incorporation into conductive material 16. Accordingly, bottom electrode 16 and top electrode 26 can comprise identical compositions as one another. Alternatively, top electrode 26 can comprise a different chemical composition than does bottom electrode 16. In particular applications, top electrode 26 will comprise, consist essentially of, or consist of one or both of tungsten and silver.

In the shown application, top electrode 26 is physically against conductive material 24. If material 26 and conductive layer 24 both comprise silver, the top electrode 26 can be formed in a separate processing step from silver-containing layer 24. Further, top electrode 26 can comprise a different silver composition than does layer 24. Specifically, layer 24 can consist of very pure silver, whereas layer 26 can comprise a sliver alloy, such, for example, a copper/silver alloy. If top electrode 26 comprises silver, it can be advantageous to omit thin layer 24 from the construction 10 of FIG. 2.

In the construction 10 of FIG. 2, layers 18 and 22 can be electrically insulative, and can be referred to as glass layers. Further, layers 18 and 22 can be significantly thinner than the layer 20 provided between layers 18 and 22. For instance, layers 18 and 22 can each be thinner than layer 20 by at least about 300 angstroms. In an exemplary application, layers 18 and 20 can each have thicknesses of at least about 150 angstroms, and the layer 20 can have a thickness of at least about 450 angstroms. In a specific application, layers 18 and 22 can each have thicknesses of about 150 angstroms, and layer 20 can have a thickness of about 470 angstroms.

Referring to FIG. 3, layers 16, 18, 20, 22, 24 and 26 are patterned into a construction 30. Such patterning can be accomplished by, for example, photolithographic processing utilized in conjunction with an appropriate etch, or series of etches, for extending a desired pattern through layers 16, 18, 20, 22, 24 and 26. In particular aspects, construction 30 can correspond to a tunnel diode construction.

Construction 30 is shown electrically connected with potentials 32 and 34 through top and bottom electrodes 16 and 26, respectively. Potential 32 is positive relative to potential 34, as indicated by the “+” and “−” symbols shown in FIG. 3. Potentials 32 and 34 can be utilized to drive current through construction 30, and accordingly operate construction 30 as part of an integrated circuit assembly. Construction 30 can be incorporated into the assembly as, for example, either a part of logic circuitry, or as part of a microwave oscillator construction. Potentials 32 and 34 can be generated by other circuitry (not shown) associated with substrate 12 and incorporated within an integrated circuit comprising construction 30.

Another aspect of the invention is described with reference to FIGS. 4-7. In referring to FIGS. 4-7, similar numbering will be utilized as was used above in describing the aspect of FIGS. 1-3, where appropriate.

Referring initially to FIG. 4, a construction 50 is illustrated at a preliminary stage of the second aspect of the invention. Construction 50 comprises substrate 12, insulative material 14, and bottom electrode 16 of the FIG. 1 construction. The construction 50 can be formed in processing subsequent to that described above with reference to FIG. 1, and accordingly can correspond to a processing stage subsequent to the stage of FIG. 1.

A material 52 is formed over bottom electrode 16. In the shown embodiment, material 52 is formed physically against bottom electrode 16. Material 52 comprises Ge and one or more of S, Te and Se. Material 52 can comprise, for example, Ge_(x)Se_(y), where “x” is from about 20 to about 30, and where “y” is from about 70 to about 80. The sum of x and y can be 100. Alternatively, Ge_(x)Se_(y), can be expressed as comprising “x” from about 0.2 to about 0.3, “y” from about 0.7 to about 0.8, and “x”+“y” equal to 1.

In particular applications, layer 52 will comprise Ge₂₅Se₇₅. In the shown application, layer 52 is formed physically against bottom electrode 16.

A conductive material 54 is formed in a layer over material 52. In the shown application, layer 54 is formed physically against material 52. Layer 54 can comprise, consist essentially of, or consist of a transition metal. In particular applications, layer 54 can comprise, consist essentially of, or consist of one or more transition metals selected from Group IB of the periodic table of the elements. For instance, layer 54 can comprise, consist essentially of, or consist of copper or silver. In an exemplary application, material 52 comprises Ge_(x)Se_(y), where “x” is from about 20 to about 30, and where “y” is from about 70 to about 80; and layer 54 comprises, consists essentially of, or consists of silver. Layer 54 can be formed to a thickness of, for example, about 100 Å and layer 52 can be formed to a thickness of, for example, about 500 Å.

Referring to FIG. 5, layer 54 is exposed to light 56 having an appropriate energy to cause at least some of the metal from layer 54 to disperse within material 52 (with the dispersed metal being indicated by stippling throughout material 52). Light 56 can comprise ultraviolet light. In an exemplary application a suitable wavelength of light 56 can be from about 200 nanometers to about 600 nanometers, which can disperse silver from layer 54 into material 52.

The amount of transition metal dispersed from layer 54 into material 52 is preferably equal to or greater than an amount which saturates material 52, but less than the amount which will crystallize separate phases of germanium-comprising constituents from within material 52. For instance, if material 52 comprises Ge₂₅Se₇₅, and layer comprises silver, the amount of silver dispersed into material 52 will preferably be sufficient to form phase-separated silver within the Ge/Se/Ag material 52 formed by dispersion of the silver into material 52. However, the amount of silver dispersed into material 52 will preferably be a low enough concentration to avoid crystallization of Ge/Se materials from within material 52. If such crystallization occurs, such can break a potential within the material and impede or destroy operation of a tunnel diode (or other device displaying differential negative resistance) comprising the material.

Material 52 and layer 54 can be formed by either sputter deposition or evaporative deposition. It is noted that even though only some of the metal of layer 54 is shown dispersed into material 52 in the shown application, the invention can encompass other applications wherein an entirety of layer 54, or substantially an entirety of the layer 54, is dispersed into material 52.

Referring to FIG. 6, a conductive material 26 is formed over layer 54. Conductive material 26 can be referred to as a top electrode, and can comprise identical components as described with reference to FIG. 2 for the material 26 of construction 10. Accordingly, material 26 can comprise, consist essentially of, or consist of Ag, W, Ti, TiN, TaN and/or Pt; and in particular embodiments can comprise, consist essentially of, or consist of one or both of Ag and W. In the shown application, material 26 is formed physically against the layer 54.

Referring to FIG. 7, layers 16, 52, 54 and 26 are patterned into an assembly 60. Such patterning can be accomplished utilizing photolithographic processing and one or more suitable etches. The assembly 60 can correspond to a tunnel diode assembly.

The bottom electrode 16 is electrically connected with a potential 32, and the top electrode 26 is electrically connected with a potential 34. The potential 32 is positive relative to the potential 34. Potentials 32 and 34 can, as described above with reference to FIG. 3, be part of an integrated circuit structure comprising assembly 60.

The construction 60 can function comparably to the construction 30 of FIG. 3. However, it may be more difficult to specifically tailor construction 60 for a particular desired operation than to tailor construction 30. However, it can, in particular applications, be easier to accomplish the fabrication of construction 60 than that of construction 30. Accordingly, both construction 60 and construction 30 can have advantages and disadvantages relative to one another.

The devices displaying differential negative resistance displaying differential negative resistance formed in accordance with aspects of the present invention can have exceptional operating characteristics. Such characteristics are described with reference to FIGS. 8 and 9. FIG. 8 illustrates a graph of current (measured in microAmps) versus voltage (measured in Volts) for a device of the type described with reference to FIG. 3, in which layers 18 and 22 comprise Ge₄₀Se₆₀, and in which layer 20 comprises Ag_((2+Δ))Se, with Δ being greater than 0. The curve of FIG. 8 demonstrates that the device has a first tunnel region (or DNR) 100 at a voltage slightly above −0.5 Volts. FIG. 8 also shows that discreet quantum sets (illustrated generally by arrows 102 and 104) occur at about −2.0 Volts and at about −3.0 Volts. The discrete quantum sets indicate that the Ag_((2+Δ))Se of the device comprises at least about 20 quantized energy levels (the number of peaks in sets 102 and 104, and which can also include the peak 100) when the layer is exposed to voltages having an absolute value of from 0 volts to 3.5 volts. In particular aspects, the material having the quantized energy levels can be considered to comprise, consist essentially of, or consist of Ag and Se. Further, the number of quantized energy levels can be considered to be, for example, at least 4, at least 6, at least 10, at least 15 and at least 20. The quantized energy levels can, in particular aspects, be considered resonances (possibly discrete or separate relative to one another) induced when the material is exposed to appropriate voltages between 0 volts and −3.5 volts (or, more generally, to appropriate voltages having an absolute value of from greater than 0 volts to 3.5 volts).

Referring to FIG. 9, the first tunnel region of a device formed in accordance with an aspect of the present invention is shown on an expanded scale relative to the scale of FIG. 8, in a graph 150 of current-versus-voltage. In accordance with convention, the device is described as having a peak (200) at about −0.3 volts and a valley (202) at about −0.6 volts. It is noted that the peak and valley are shown inverted in the graph of FIG. 9, but convention would be to flip the graph so that the peak-to-valley ratio is greater than 1. The shown device has a peak-to-valley ratio of about −50 microAmps/−0.4 microAmps, or in other words, about 130. The shown device is an exemplary device, and various other devices displaying differential negative resistance have been formed utilizing methodology of the present invention. Such devices have been found to have peak-to-valley ratios greater than 8, and specifically to have peak-to-valley ratios of at least 9, at least about 10, at least about 20, at least about 30, at least about 40, at least about 50, at least about 75, at least about 100, and greater than at least about 100 as demonstrated by FIG. 9. It is found that the peak-to-valley ratios of devices of the present invention typically do not vary significantly with temperature, and specifically that the ratios stay relatively constant across a temperature range of from about −30° C. to about 100° C.

The device of FIG. 9 has both the peak in current value (200) and the valley in current value (202) occurring within a voltage range of from less than 0 volts to greater than −1 volt, or in other words, occurring within a voltage range having an absolute value of from greater than 0 volt to less than 1 volt.

The assemblies of the present invention can have numerous applications, including, for example, applications in logic devices, and applications in microwave oscillator devices.

In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents. 

1-83. (canceled)
 84. A method of forming an assembly displaying differential negative resistance, comprising: providing a semiconductor substrate; forming a first layer comprising Ge and one or more of S, Te and Se over the semiconductor substrate; forming a second layer over the first layer, the second layer comprising M and A, where M is a transition metal and A is one or more of 0, S, Te and Se; and forming a third layer over the second layer, the third layer comprising Ge and one or more of S, Te and Se.
 85. The method of claim 84 wherein the first and third layers are formed by sputter deposition.
 86. The method of claim 84 wherein the first and third layers are formed by evaporative deposition.
 87. The method of claim 84 wherein the second layer is formed by sputter deposition.
 88. The method of claim 84 wherein the second layer is formed by evaporative deposition.
 89. The method of claim 84 wherein M and A are present as M_((2+Δ))A, where Δ is greater than
 0. 90. The method of claim 89 wherein the transition metal is selected from Group IB of the periodic table of the elements.
 91. The method of claim 90 wherein the transition metal is copper.
 92. The method of claim 90 wherein the transition metal is silver.
 93. The method of claim 84 where A is
 0. 94. The method of claim 84 where A is S.
 95. The method of claim 84 where A is Te.
 96. The method of claim 84 where A is Se.
 97. The method of claim 84 further comprising: forming an electrically conductive bottom electrode over the substrate, and forming the first layer over the bottom electrode; forming a silver-containing layer over the third layer; forming an electrically conductive top electrode over the silver containing layer; and wherein the second layer is formed to be physically against the first layer, and the third layer is formed to be physically against the second layer.
 98. The method of claim 97 wherein the silver-containing layer consists essentially of silver.
 99. The method of claim 97 wherein the first layer is formed to be physically against the bottom electrode, and wherein the silver-containing layer is formed, to be physically against the third layer.
 100. A method off forming an assembly displaying differential negative resistance, comprising: providing a semiconductor substrate; forming a first layer comprising Ge₄₀Se₆₀ over the semiconductor substrate, forming a second layer comprising Ag_((2+Δ))Se over the first layer, where Δ is greater than 0; and forming a third layer comprising Ge₄₀Se₆₀ over the second layer.
 101. The method of claim 100 wherein the first, second and third layers are formed by sputter deposition.
 102. The method of claim 100 wherein the first, second and third layers are formed by evaporative deposition.
 103. The method of claim 100 wherein the second layer is formed to be physically against the first layer, and wherein the third layer is formed to be physically against the second layer.
 104. The method of claim 100 further comprising: forming an electrically conductive bottom electrode over the substrate; forming the first layer over the bottom electrode; forming a silver-containing layer over the third layer; forming an electrically conductive top electrode over the silver containing layer; and wherein the second layer is formed to be physically against the first layer, and the third layer is formed to be physically against the second layer.
 105. The method of claim 104 wherein the silver-containing layer consists essentially of silver.
 106. The method of claim 104 wherein the first layer is formed to be physically against the bottom electrode, and wherein the third layer is formed to be physically against the silver-containing layer.
 107. The method of claim 104 wherein the first layer is formed to be physically against the bottom electrode; wherein the third layer is formed to be physically against the silver-containing layer; and wherein the top electrode is formed to be physically against the silver-containing layer.
 108. The method of claim 104 wherein: the first layer is formed to be physically against the bottom electrode, the bottom electrode comprising one or more of Ag, W, Ti, TiN, and Pt; the third layer is formed to be physically against the silver containing layer; and the top electrode is formed to be physically against the silver containing layer, the top electrode comprising one or more of Ag and W.
 109. A method of forming an assembly displaying differential negative resistance, comprising: providing a semiconductor substrate; forming a material comprising Ge and one or more of S, Te and Se over the substrate; forming a layer comprising a transition metal over the material; exposing the layer to light to cause at least some of the transition metal to disperse into the material; and incorporating the material having the transition metal dispersed therein into an assembly displaying differential negative resistance.
 110. The method of claim 109 wherein the material is formed by sputter deposition.
 111. The method of claim 109 wherein the material is formed by evaporative deposition.
 112. The method of claim 109 wherein the layer is formed by sputter deposition.
 113. The method of claim 109 wherein the transition metal is selected from Group IB of the periodic table of the elements.
 114. The method of claim 113 wherein the transition metal is copper.
 115. The method of claim 113 wherein the transition metal is silver.
 116. The method of claim 109 wherein the material comprises S.
 117. The method of claim 109 wherein the material comprises Te.
 118. The method of claim 109 wherein the material comprises Se.
 119. The method of claim 109 wherein the layer consists essentially of the transition metal.
 120. The method of claim 109 further comprising: forming an electrically conductive bottom electrode over the substrate; forming the material over the bottom electrode; and forming an electrically conductive top electrode over the layer comprising the transition metal.
 121. A method of forming an assembly displaying differential negative resistance, comprising: providing a semiconductor substrate; forming a material comprising Ge_(x)Se_(y) over the semiconductor substrate, where x is from about 20 to about 30, and wherein y is from about 70 to about 80; forming a layer comprising Ag over the material; and exposing the layer to light to cause at least some of the Ag to disperse into the material.
 122. The method of claim 121 wherein the layer consisting essentially of Ag is formed physically against the material.
 123. The method of claim 122 further comprising: forming an electrically conductive bottom electrode over the substrate-forming the material over the bottom electrode; and forming an electrically conductive top electrode over the layer consisting essentially of Ag.
 124. The method of claim 123 wherein the material is formed physically against the bottom electrode, and wherein the top electrode is formed physically against the layer consisting essentially of Ag.
 125. The method of claim 123 wherein, the material is formed physically against the bottom electrode, the bottom electrode comprising one or more of Ag, W, Ti, TiN, and Pt; and the top electrode is formed physically against the layer consisting essentially of Ag, the top electrode comprising one or more of Ag and W. 